Mathematical modeling of oxygen exchange and transport in air-perovskite-YSZ interface regions .1. Reduction of intermediately adsorbed oxygen

The transport of oxygen in a porous perovskite solid oxide fuel cell cathode is modeled by use of the principles of porous electrode modeling, by taking into account exchange kinetics at the gas/electrode interface, bulk diffusion of oxygen vacancies, surface diffusion of adsorbed oxygen atoms, and electrochemical kinetics at the cathode/electrolyte interface. The mechanism for the latter is based on the assumption that intermediately adsorbed oxygen atoms are reduced at the cathode/electrolyte interface in favor of direct exchange of oxygen vacancies. The significance of concentration polarization is demonstrated even at very low overpotentials, especially if the adsorption process is slow. Under such conditions, the empirical correlation R-p(eff) proportional to p(O2)(-m) claimed to exist between the measured potential resistance and the partial pres sure of oxygen cannot be justified on fundamental grounds. A limiting current is obtained at high cathodic overpotentials due to the depletion of intermediately adsorbed species at the cathode/electrolyte interface. The existence of a correlation i(lim) proportional to p(O2)(n) is predicted, where the exponent n is determined by the kinetic and transport properties of the cathode for oxygen exchange and transport.